The largest exhaust valve was on some of the earlier-to-middle 1960s RB engines. BUT the main issue with "as large as I can go" valve sizing, intake or exhaust, is having those bigger valves NOT be shrouded by the combustion chamber walls and/or the cylinder bores.
It can be far better to have better-flowing ports to start with, even with a smaller valve size, rather than humongous valves themselves.
Unless the type of "competition" you plan to engage is requires a cast iron head, then an aftermarket aluminum head (of your choosing) can be a better option. Rather than focus exclusively on "top lift flow", look for heads which have better low-lift flow instead. Flow numbers which look good up top, might have ports which flow poorly at lower valve lifts on their way to those high numbers up top. Not unlike focusing only on 7000rpm horsepower while ignoring 3000rpm torque, where flow velocity AND volume make things happen at the lower rpm levels. So, be careful in looking at the flow figures and aftermarket dialogue.
THEN match the flow with great combustion chamber dynamics. Which can also relate to what the piston crown looks like.
THEN also consider rocker arm ratio. NOT to increase the valve lift, BUT to get the valve head off of the seat sooner, so port flow initiates sooner.
Search David Vizard's YouTube channel for explanations of these things AND others related to cyl head issues related to higher horsepower production. Some of his orientations might not be a
"trick of the week" sort of thing, but much more solid in their basis and results.
Remember, it's "the total package" rather than just "a combination of parts" that make great performing engines.
Enjoy!
CBODY67
